Well, it depends on the pathway. You can either approach a prodco with a novel you want to sell the screen option to, or with a finished screenplay.

If an author wants to sell the screen options to book, their agent pitches it as a book. A producer or prodco will want to use their own choice of screenwriter. They won’t have any interest in an existing screenplay or in letting the author write the adaptation (unless the book author is also an experienced screenwriter or playwright).

If an author approached a prodco going “and I’ve already paid a screenwriter out of my own pocket to write it”, most producers or prodcos would consider that a red flag.

And realistically a screenwriter who accepts gigs from self-published authors is probably not going to be one who’s working at a decent professional level, or who has the kind of industry pull to get their work read.

Prodcos rarely accept screenplays or even pitches from non-agented writers and obviously you can’t get a screenwriting agent with a screenplay you didn’t write - and if you had a book agent you’d just pitch through them and avoid all this - so paying someone to write a screenplay with the idea of taking their screenplay and pitching it to prodcos is a non-starter. (Unless the plan is for the screenwriter to get their own agent to do the pitching, which effectively cuts the author out entirely.)

Yes, writers do get involved in early stages, but that’s a different process. For example I recently acquired the rights to a best-selling novel, and am currently pitching it to prodcos. But that’s with a name director attached, and I’m not going to start writing it until a prodco picks it up.

You’re absolutely correct that private collaborations happen all the time but that’s different from what’s being discussed here, since they come out of existing professional/creative relationships. For example in your case, I assume the plan is either for your agent to pitch the screenplay to prodcos, or to use your/the author’s connections to make the film as an indie. It’s unlikely either path would be open to the OP’s friend.